Displacement current sensor

ABSTRACT

A displacement current sensor for generating electrical signals representative of relative motion between a conductor and an electric field source. 
     The sensor includes means for generating an electric field and electrically conductive means positioned for interaction with the electric field, the field generating means and the electrically conductive means being relatively movable so that variations in the field encountered by the electrically conductive means induces a displacement current, as opposed to a conduction current, in the electrically conductive means. 
     The field generating means preferably is fabricated from an electret formed in any one of a wide variety of geometrical shapes, such as planar, cylindrical or special geometrical configurations; the electrically conductive means may comprise one or more plates, wires, rods, or other special shapes.

BACKGROUND OF THE INVENTION

This invention relates to sensors of the type used to generateelectrical signals in response to relative motion between two or moresensor elements.

Many types of sensing devices are known which are capable of generatingelectrical signals in response to the relative motion between two ormore sensor elements. Examples of such devices are electromagnetictransducers, in which relative motion between an electrically conductivecoil and a magnet produces electrical current signals; electrettransducers in which relative motion between an electret member andanother member produces an electrical current; piezoelectrictransducers, triboelectric transducers and strain gauges. Allelectromagnetic transducers function in accordance with the principlesenunciated by James Clerk Maxwell, usually expressed in the form of thewell known Maxwell's Electromagnetic Equation: ##EQU1## where μ₀ is thepermeability of air, ε₀ is the permittivity of air, φ_(E) is theelectric field, β is the magnetic field, and l is the length of a closedloop ohmic conductor. The quantity i is the conduction current flowingin the conductor l, while the quantity [ε₀ (dφ_(E) /dt)] is termed thedisplacement current. This equation illustrates the interrelationshipbetween a magnetic field and two electrical quantities: viz., theconduction current and the displacement current; and shows that achanging electric field acts as a source for a magnetic field in exactlythe same manner as the conduction current which corresponds to chargesactually moving along an ohmic conductor. The displacement current hasthe dimensions of a real current even though actual charges are nottransported along an ohmic conductor. Thus, a magnetic field may beestablished in two ways: Firstly, by a changing electric field (thedisplacement current term); and secondly, by a conduction current (theconduction current term).

In all known electromagnetic sensors, in which a changing magnetic fieldis used to generate the electrical current signal, only the conductioncurrent is sensed, since the magnitude of the displacement current isnegligible when compared to that of the conduction current. In all othertransducers of the class described above, the current produced by thetransducer is also a conduction current.

While transducers which employ the conduction current are generallyquite useful, certain limitations inhere in any transducer of this type.Since such transducers require a closed loop ohmic circuit for theconductive current, any varying electromagnetic radiation in thevicinity of the closed loop conductive path (which is usually coupled toamplifying the measuring circuitry) produces spurious conductive currentsignals which are capable of masking the information conveyed in theconductive current signals generated by the transducer. Accordingly,great care is required to shield the conductive loop portion of suchsensors against stray electromagnetic radiation. Such shields introducecomplexity to the structure of the transducers themselves and also tothe structure of the conductive paths used to couple the transducer tothe amplifying and measuring circuitry. In addition, with the exceptionof the electromagnetic transducer noted above, conductive currentsensors require the application of electrical power to the closed loopportion of the circuit in order for the device to be operable. Whilethis requirement poses no constraint in many applications, there areother applications for which this requirement is extremely burdensome.For example, in applications requiring the installation of many sensorsin a structure for the purpose of monitoring vibrations of thestructure, either individual sources of electrical power must beinstalled at the site of each sensor or the structure must be wired toprovide electrical power from a central source to the individualsensors. In an office high-rise building, for example, in which it isdesired to install several sensors on each floor of the building toprovide vibration information, this requirement is at best costly and atworst impossible for pre-existing structures. In aircraft sensingapplications, this requirement is similarly highly undesirable.

In addition to the above-noted disadvantages, conductive current sensorswhich employ permanent magnets possess additional disadvantages. Forexample, the energy required to magnetize a permanent magnet isrelatively high when compared to the energy required to polarize anelectret substance. Further, in sensor applications requiring geometrywhich is tailored to the geometry of the structural member whose motionor vibration is to be sensed, it can be quite costly and difficult toprovide the necessary geometry for the permanent magnets. In addition,permanent magnets possess aging characteristics which result in areduction of the magnetic field strength provided by the magnet withtime, which requires recalibration of the system composed of theindividual sensors at predetermined intervals and eventual replacementof the magnets.

SUMMARY OF THE INVENTION

The invention comprises a displacement current sensor which employs anelectrically conductive gaussian surface to detect the time rate ofchange of the electric field generated by charges, i.e., thedisplacement current induced in the gaussian surface by a varyingelectric field, the sensor being immune to electromagnetic radiation andrequiring no application of electrical power.

Broadly, the invention comprises a displacement current sensor forgenerating electrical signals representative of relative motion betweena conductor and an electric field source, the sensor including means forgenerating an electric field and electrically conductive meanspositioned for interaction with the electric field, the field generatingmeans and the electrically conductive means being relatively movable sothat variations in the field encountered by the electrically conductivemeans induces a displacement current, as opposed to a conductioncurrent, therein. Preferably, the field generating means comprises anelectret which may be formed in a wide variety of geometrical shapes,such as planar, cylindrical or special geometrical shapes, while theelectrically conductive means may comprise plates, wires, rods, or otherspecial shapes.

In the simplest planar embodiment, the field generating means comprisesan electret with a planar shape and the electrically conductive meanscomprises a conductive plate having a surface generally parallel to theplane of the electret but spaced therefrom. In an alternate embodiment,the electrically conductive means comprises a flat spiral coil generallyparallel and spaced from the plane of the electret. In other alternateembodiments, multiple electret plates are spaced in a direction normalto the planar surfaces and a plurality of electrically conductive wiresor plates are received within the spaces between adjacent electricplates. In still other alternate embodiments, first and secondoppositely polarized planar electret members are mutually spaced in adirection normal to the plane of each member, and an electricallyconductive rod, wire or plate is received within the space between theelectret members. In still another planar embodiment, first and secondoppositely polarized generally planar electret members are mutuallyspaced in a direction normal to the plane of each member, anelectrically non-conductive member fabricated from a dielectric oranother electret is received within the space between the first andsecond electret members, and an electrically conductive means is coupledto one of the first and second electret members on the opposite surfacethereof from the space, the electrically conductive means providing agaussian surface for generating an induced displacement current inresponse to variations in the electric field between the first andsecond electret members caused by relative motion between these membersand the electrically non-conductive interstitial member.

In the simplest embodiment of the invention employing cylindricalgeometry, the field generating means comprises a cylindrical electretmember, while the electrically conductive means comprises a conductiverod received within the inner diameter of the electret. Alternatively,the electrically conductive means comprises a coil and the fieldgenerating means comprises an electret member in the shape of a rod, therod being received within the inner diameter of the coil.

Each embodiment may be provided with an electrostatic shield to preventinteraction between the field generated by the field generating meansand stray electric fields. In addition, all embodiments may be coupleddirectly to a suitable measuring instrument, such as an electrometer, ormay be coupled to the input of an amplifier, which may be either localto the sensor or remote therefrom, and having a relatively low inputimpedance and negligible zero drift.

All embodiments of the invention are capable of affording highreliability at extremely low cost and light weight and thus are suitablefor a wide variety of applications.

For a fuller understanding of the nature and advantages of theinvention, reference should be had to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a first embodiment of theinvention employing planar geometry;

FIG. 2 is a schematic diagram of the invention using multiple plategeometry;

FIG. 3 is a schematic diagram of an alternate embodiment of theinvention using cylindrical geometry;

FIG. 4 is an alternate embodiment of the invention also usingcylindrical geometry;

FIGS. 5-9 illustrate still other embodiments of the invention; and

FIG. 10 is a circuit schematic of an amplifier circuit for use with theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

All embodiments of the invention function by measuring the Maxwelldisplacement current generated whenever an electric field changes withtime. With reference to FIG. 1, an electrically conductive plate 11 isnormally spaced by a distance d from an electric field source 12, whichmay comprise a charged plate, an electret planar member or otherequivalent devices. An ohmic conductor 13 is coupled between the plate11 and a measuring instrument 15 such as an ammeter, an electrometer orthe equivalent. Whenever the separation distance d between plate 11 andfield source 12 changes, the magnitude of the electric field encounteredby the conductive plate 11 changes which produces a displacement currentgiven by the equation:

    i.sub.d =ε.sub.0 (dφ.sub.E /dt)

From Gauss' law it is known that:

    E=(q/ε.sub.0 A)

where A is the surface area of plate 12 and q is the charge present onor in field source 12. By differentiating the above expression: ##EQU2##By combining this expression with the expression for Maxwell'sdisplacement current: ##EQU3## that is, the total current is simply thedisplacement current. Since the electrical field in the space betweenplate 11 and electret member 12 is inversely proportional to theseparation distance, variation of this distance d causes the generationof displacement current which can be measured directly by the measuringinstrument 15. Alternatively, the displacement current may be generatedby varying the magnitude of the area A of plate 11, by changing thestrength of the charge q on or in field source 12 or by altering thepermittivity ε₀ of the region between plate 11 and source 12 byinserting a dielectric substance having a different permittivity thanair. As an example of the magnitude of displacement current which can begenerated, for a circular plate 11 and a circular electret member 12each having a diameter of 10 centimeters and a changing electric fieldof 10¹² volts per meter-second, the displacement current is 0.070 amp,which is a very detectable magnitude.

The magnitude of the displacement current may be increased by using thearrangement of FIG. 2 in which a plurality of conductive plates 11 arereceived within the spaces between adjacent ones of a plurality of fieldgenerating plates 12, the conductive plates 11 being coupled in parallelto lead 13 and thus to the measuring instrument 15. Alternatively, lead13 may be coupled to the input of the amplifier shown in FIG. 10,described below.

It should be noted that the geometry of the plate 11 and the electricfield generating element 12 may assume any suitable configuration, suchas circular, rectangular, square or special configurations required inany given application of the sensor. While in the FIG. 1 embodiment thedisplacement current may be generated by varying either the separationdistance d or by translating plate 11 or field generating element 12 inthe vertical plane as illustrated in order to vary the effective area ofthe plate 11, in the FIG. 2 embodiment motion is restricted tohorizontal movement as depicted.

FIG. 3 illustrates a cylindrical embodiment of the invention in whichthe field generating element comprises a cylindrical electret member 31and the conductive member comprises a conductive rod 32 having an outerdiameter which is less than the inner diameter of the electret member31. In this embodiment, an electrostatic shield 33 fabricated from asuitable material such as aluminum foil is placed about the electretmember 31 and coupled to a reference potential 34, such as ground, inorder to shield the sensor from the effects of stray electrostaticfields. It should be noted that a major advantage of sensors fabricatedin accordance with the invention is the fact that such sensors arecompletely immune to electromagnetic radiation, since the electricallyconductive member 11, 32 is connected in an open loop configuration.

FIG. 4 illustrates an alternate cylindrical embodiment of the inventionin which the electret comprises a solid cylindrical rod 41 and theelectrically conductive member comprises a coil 42 having an innerdiameter greater than the outer diameter of the rod 41.

In both the FIG. 3 and FIG. 4 embodiments, relative axial motion, ortorsional motion, of either of the elements results in the generation ofthe displacement current.

FIG. 5 illustrates an alternate planar embodiment of the invention inwhich the electrically conductive member comprises a flat spiral coil51. In this embodiment, the displacement current may be generated eitherby motion in a plane parallel to the coil 51 or field generating plate52 or by motion perpendicular to such planes.

FIG. 6 illustrates an alternate embodiment of the invention in which theelectric field generating means comprises a pair of spaced plates 61, 62bearing charges of opposite polarity, and in which the electricallyconductive member comprises an interstitial plate 63 arranged formovement in the direction of the vertically depicted arrow 64.

FIG. 7 shows a variation of the FIG. 6 arrangement in which theelectrically conductive member comprises a wire or rod 71 arranged formotion in the direction of the horizontally depicted arrow 72.

FIG. 8 shows an alternate embodiment of the invention employing planargeometry in which oppositely charged field generating plates 81, 82coact with a member generally designated by reference numeral 83 andcomprising a sandwich fabricated from oppositely polarized dielectriclayers 84, 85 and intermediate electrically conductive plate 86.

FIG. 9 illustrates an alternate embodiment of the invention in which aspaced pair of oppositely polarized electret plates 91, 92 coact with atranslatable electrically non-conductive interstitial plate fabricatedfrom a dielectric material or an electret. The electrically conductivemember in this embodiment comprises a conductive layer 94 adhered to thesurface of electret member 91 on the opposite surface from that definingthe gap.

While the field generating elements 12, 31, 41, 52, 61, 62, 81, 82, 91,and 92 may comprise any one of a number of elements capable ofestablishing the electrostatic field required, such as charge bearingconductive plates, these elements are preferably fabricated from any oneof a number of suitable electret substances which are permanentlypolarized. Examples of such electrets may be taken from the class ofpolymer electrets, which are polymers consisting of microcrystalsembedded in an amorphous matrix and whose composition varies from a fewpercent up to 90%. Examples of such polymer electrets are acrylics,ethylcellulose, polystyrene, vinyl polymers, polyurethane, and materialssold under the trademark "MYLAR" or "TEFLON". Fabrication of electretsis a well known process which generally proceeds by heating the electretmaterial to a temperature just below the melting point in the presenceof an electric field, and subsequently permitting the electret to cool,also in the presence of the field.

So called monocharge electrets are highly suitable for use infabricating sensors according to the invention. These electrets are thinfilm electrets having a single charge, either positive or negative.Electrets of this type fabricated from TEFLON have a thickness in therange from 12 to 25 microns and have been charged to a charge density inthe range from about 10 to about 20 nanocoulombs per square meter. Suchelectrets are capable of providing a vibrational frequency response ofextremely broad bandwidth in the range from about 10⁻³ to about 2×10⁸Hz. Further, such electrets have a longer charge lifetime due to theabsence of recombination losses and the possibility of a larger gapseparation with comparable electric flux than other types of electrets.

As noted above, in some applications of the invention it may be desiredto amplify the displacement current signals produced by sensorsfabricated according to the invention. FIG. 10 is a schematic of anamplifier circuit which is suitable for such use. As shown in thisfigure, an operational amplifier 101, preferably a Burr-Brown type Model3430 amplifier, has a first input which is coupled via input conductor13 to electrically conductive plate 11 and a second input which iscoupled to reference potential, such as ground. Amplifier 101 isconfigured by means of resistances R1, R3 and R4 as a current feedbackcurrent-to-voltage transformation mode amplifier, and is capable ofmeasuring extremely small currents on the order of 1 picoampere (10⁻¹²amp). For the Burr-Brown type of amplifier noted above, the operationalcharacteristics are as follows:

    ______________________________________                                        Open loop gain (A)  100 db                                                    Input bias current (I.sub.b)                                                                      10 × 10.sup.-15 amp                                 Input current noise (I.sub.n)                                                                     1 × 10.sup.-15 amp                                  Input offset voltage (V.sub.os)                                                                   30 × 10.sup.-6 volt/°C.                      Input noise voltage (E.sub.n)                                                                     10 × 10.sup.-6 volt/°C.                      High input resistance                                                                             1 × 10.sup.14 ohms                                  ______________________________________                                    

For a feedback resistance of 10¹² ohms, a displacement current of 1picoamp will produce a signal of 1 volt at the output of the circuit.With a resolution of 1 millivolt, an input current resolution of 10⁻¹⁵amp is possible. Preferably, the value of (R2+R3/R3) should be less than100 in order to restrict noise error voltages. The dependence of zerodrift on the ratio of R_(f) to the equivalent internal impedance of thesensor leads to the limitation on the value of R_(f) to a value nolarger than the source impedance. However, since the sensor sourceimpedance is that of an open loop circuit, very large values of R_(f)can be used without incurring drift problems. The advantage of employinga relatively high open loop gain comprises the corresponding reductionof the equivalent operational amplifier input impedance by a factor of(1/A). This relatively low impedance employed in low currentmeasurements assures measurement accuracies and reduces insulationrequirements for the components. The input capacitance is also reducedby a factor of (1/A), so that the measurement rise time is no longer afunction of the input RC time constant and becomes a function of R_(f)×C_(f) where C_(f) is the shunt capacity of R_(f). Thus, thedisplacement current, although very small in magnitude, may be readilyconverted to usable voltages with the circuit of FIG. 10. In addition,the amplifier circuitry can be located either at the sensor or at aremote site, because of the principle of the continuity of current,i.e., the value of the displacement current is the same at each point inthe circuit.

Sensors fabricated according to the invention are ideally suited for awide variety of applications, due to their low cost, light weight, noiseimmunity, simplicity of construction, and adaptability to differentgeometrical configurations. Such sensors, for example, may be used tomeasure linear or torsional displacements of structural members, such assupport beams in buildings, aircraft struts; to measure ultrasonicvibrations in liquids and solids; in crystalography to measure stress,strain, crystal growth and cleavage; in the field of bionics to measurechanging electric fields from living organs, such as the human centralnervous system or human brain; in atmospheric physics to measureatmospheric currents; in acoustic holography to measure signaltransmission modes in dielectrics and conductors; and in otherapplications which will occur to those skilled in the art.

When fabricated using electret materials, the sensors are also energyefficient in that the energy required to establish a uniform electricfield required to polarize the electret is substantially less than theenergy required to set up a comparable uniform magnetic field requiredto orient a permanent magnet.

While the above provides a full and complete disclosure of theinvention, various modifications, alternate constructions andequivalents may be employed without departing from the true spirit andscope of the invention. Therefore, the above description andillustrations should not be construed as limiting the scope of theinvention, which is defined by the appended claims.

What is claimed is:
 1. A displacement current sensor for generatingelectrical signals representative of relative motion between a conductorand an electric field, said sensor comprising:means for generating anelectric field; electrically conductive means positioned for interactionwith said electric field, said field generating means and saidelectrically conductive means being relatively arranged in such a mannerthat variations in the field encountered by said electrically conductivemeans induce a displacement current in said electrically conductivemeans; and single electrically conductive output terminal means coupledto said electrically conductive means for manifesting said displacementcurrent.
 2. The combination of claim 1 further including an electricshield means for preventing interaction between said electric field andstray electric fields.
 3. The combination of claim 1 further includingamplifier means having an input coupled to said electrically conductivemeans for amplifying said displacement current.
 4. The combination ofclaim 1 wherein said field generating means comprises an electret. 5.The combination of claim 4 wherein said electret has a planar shape andsaid electrically conductive means comprises a wire.
 6. The combinationof claim 5 wherein said wire is formed as a flat spiral coil generallyparallel to the plane of said electret.
 7. The combination of claim 4wherein said electret has a planar shape and said electricallyconductive means comprises a conductive plate having a surface generallyparallel to the plane of said electret.
 8. The combination of claim 4wherein said electret has a cylindrical shape and said electricallyconductive means comprises a conductive rod received within the innerdiameter of said electret in non-contacting relationship.
 9. Thecombination of claim 8 wherein said electret is surrounded by anelectrically conductive shield.
 10. The combination of claim 4 whereinsaid electrically conductive means comprises a coil and said electrethas the shape of a rod, said rod being received within the innerdiameter of said coil in non-contacting relationship.
 11. Thecombination of claim 1 wherein said field generating means comprises aplurality of generally planar electret members mutually spaced in adirection normal to the plane of each member, and wherein saidelectrically conductive means comprises a plurality of electricallyconductive members each received with a different space between adjacentelectret members, said electrically conductive members being arranged innon-contacting relationship with said electret members.
 12. Thecombination of claim 11 wherein said electrically conductive members arewires.
 13. The combination of claim 11 wherein said electricallyconductive members are plates.
 14. The combination of claim 1 whereinsaid field generating means comprises a first pair of oppositelypolarized generally planar electret members mutually spaced in adirection normal to the plane of each member, and wherein saidelectrically conductive means comprises an electrically conductivemember received within the space between said electret members innon-contacting relation therewith.
 15. The combination of claim 14wherein said electrically conductive member is a wire.
 16. Thecombination of claim 14 wherein said electrically conductive member is aplate.
 17. The combination of claim 14 further including a second pairof oppositely polarized generally planar electret members coupled tosaid electrically conductive member, the polarity of each of said secondpair of oppositely polarized electret members being opposite to thepolarity of the facing one of said first pair of oppositely polarizedelectret members.
 18. A displacement current sensor for generatingelectrical signals representative of relative motion between a conductorand an electric field, said sensor comprising:first and secondoppositely polarized generally planar electret members mutually spacedin a direction normal to the plane of each member; an electricallynon-conductive member received within the space between said first andsecond electret members and arranged for relative motion therewith innoncontacting relationship; electrically conductive means coupled to oneof said first and second electret members on the opposite surfacethereof from said space and providing a gaussian surface for generatingan induced displacement current in response to variations in theelectric field between said first and second electret members; andsingle electrically conductive output terminal means coupled to saidelectrically conductive means for manifesting said displacement current.19. The combination of claim 18 wherein said electrically non-conductivemember comprises a dielectric.
 20. The combination of claim 18 whereinsaid electrically non-conductive member comprises an electret.
 21. Thecombination of claim 18 wherein said electrically conductive meanscomprises a flat member adhered to said opposite surface.